Ecteinascidin-743 (ET-743) is a novel, low molecular weight, anti-neoplastic drug which possesses extremely potent cytotoxicity against human cancer cell lines and human xenografts. The drug is particularly useful against a variety of sarcomas which generally lack alternative chemotherapeutic options. Phase I studies have demonstrated objective responses and phase II studies are currently underway. Despite the promise that this drug offers, its molecular target remains to be elucidated. ET-743 is known to decrease the rate of transcription of the MDRl (P- glycoprotein) gene. The data presented here indicate that the orphan nuclear receptor SXR can activate transcription of the MDRl. This led us to test whether the transcriptional inhibitory effects of ET-743 may be mediated be SXR. Indeed, ET-743 inhibits transactivation of SXR (IC50 = 5 nM) at the same concentrations that it acts as an antitumor agent. These data suggest that SXR is a molecular target for ET-743 and further imply that SXR, a modulator of MDRl, is a potential target for anti-tumor therapies. ET-743 is a tetrahydroisoquinoline alkaloid isolated from the marine tunicate Ecteinascidia turbinata and has been difficult to purify or synthesize in bulk quantities. The identification of a nuclear receptor as a molecular target for ET-743 would provide a rapid, high-throughput assay for the screening of alternative synthetic or natural product inhibitors. We propose to further validate the role of SXR as an anti-tumor target, to delineate the mechanism by which ET-743 inhibits SXR, and to develop assays that can later be utilized for high-throughput screening of low molecular weight chemical libraries.